Support OnHttpRequest Lua callback

If your redbean `/.init.lua` file defines a global callable named
`OnHttpRequest` then redbean will delegate all serving control to
your function. You may then restore the default serving paths, by
calling the new `Route()`, `RouteHost()`, and `RoutePath()` APIs.

Closes #150

tool/net/demo/redbean-xhr.lua

@@ -0,0 +1,8 @@
+-- redbean xhr handler demo
+hdr = GetHeader('x-custom-header')
+if hdr then
+   SetHeader('Vary', 'X-Custom-Header')
+   SetHeader('X-Custom-Header', 'hello ' .. hdr)
+else
+   ServeError(400)
+end